Hydrostatic transmission

ABSTRACT

A hydrostatic transmission comprises a housing whose interior space serves as a fluid sump, a pair of first and second fluid passages disposed in the housing, the pair of first and second fluid passages interposed between the hydraulic pump and the hydraulic motor so as to constitute a closed fluid circuit of an HST. A charge fluid passage is disposed in the housing so as to be connected with each of the first and second fluid passages for supplying fluid from the fluid sump into the closed fluid circuit. A drain fluid passage including an orifice is disposed in the housing so as to be connected with at least one of the first and second fluid passages so that, when hydraulic pressure in the at least one of the first and second fluid passages is increased beyond its neutral level, excessive fluid in the at least one of the first and second fluid passages is drained through the drain fluid passage to the fluid sump, thereby expanding the neutral zone of the HST. The charge fluid passage and the drain fluid passage are open to the fluid sump while the charge fluid passage and the drain fluid passage being separated from each other, thereby preventing the closed fluid circuit from overheating. If a center section, which has a first and second side ends opposite to each other and forms the first and second fluid passages therein, is disposed in the housing, an opening of said charge fluid passage in communication with said fluid sump is disposed toward said first side end of said center section, and an opening of said drain fluid passage in communication with said fluid sump is disposed toward said second side end of said center section.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a hydrostatic transmission (hereinafter, “HST”).

[0003] To be detailed, it relates to a technique for preventing a hydraulic circuit of the HST from overheating.

[0004] 2. Related Art

[0005] Conventionally, there is a well-known HST having its neutral position expanded.

[0006] For example, there is a conventional transaxle apparatus equipped with a power transmission system including a conventional HST 8 for enabling a vehicle to travel forwardly and backwardly, as shown in FIG. 1. HST 8 comprises a variable displacement hydraulic pump 11 including a movable swash plate 13 and a fixed displacement hydraulic motor 21. Hydraulic pump 11 and motor 21 are fluidly connected with each other through a closed circuit comprising a first oil passage 93 and a second oil passage 94. Second oil passage 94 is hydraulically pressurized higher than first oil passage 93 during backward traveling. A charge oil passage 82 is provided for supplying the closed circuit with oil. Between charge oil passage 82 and each of first and second oil passages 93 and 94 is interposed a check valve 83. Check valve 83 provided for second oil passage 94 is bypassed by a drain oil passage including an orifice 116. Even if hydraulic pump 11 is inaccurately located in neutral, that is, swash plate 13 is slightly moved from its proper neutral position into its backward traveling range, increased pressure oil is drained from second oil passage 94 through orifice 116 so as to keep the hydraulic pressure in second oil passage 94 at its neutral level, thereby preventing a vehicle from jerking caused by unexpected driving of hydraulic motor 21.

[0007] Thus, movable swash plate 13 is provided in its movable range with a dead zone expanded from its proper neutral position into its backward traveling range for keeping hydraulic motor 21 neutral. For attaching the transaxle apparatus having HST 8 shown in FIG. 1 onto a vehicle, swash plate 13 is only required for sufficiently interlocking with a manual speed control tool such as a lever or a pedal to be positioned within the dead zone while the speed control tool is set at its neutral position, thereby easing an adjustment job before the releasing of the vehicle equipped with the transaxle apparatus.

[0008] However, actually, the operating oil drained from second oil passage 94 through the drain oil passage is not returned directly into an oil sump 9 but is returned into charge oil passage 82. Hence, the heated oil drained from second oil passage 94 is absorbed through check valve 83 into hydraulically depressed first oil passage 93 while being insufficiently cooled, thereby increasing the temperature of operating oil circulated between first and second oil passages 93 and 94 and reducing the displacement efficiency of HST 8.

SUMMARY OF THE INVENTION

[0009] An HST concerning the present invention is essentially constructed as follows: a housing, whose interior space serves as a fluid sump, contains a pair of first and second fluid passages, a hydraulic pump and a hydraulic motor. The first and second fluid passages are interposed between the hydraulic pump and the hydraulic motor so as to constitute a closed fluid circuit of the HST. A charge fluid passage is disposed in the housing so as to be connected with each of the first and second fluid passages for supplying fluid from the fluid sump into the closed fluid circuit. A drain fluid passage including an orifice is disposed in the housing so as to be connected with at least one of the first and second fluid passages so that, when hydraulic pressure in the at least one of the first and second fluid passages is increased beyond its neutral level, excessive fluid in the at least one of the first and second fluid passages is drained through the drain fluid passage to the fluid sump, thereby expanding the neutral zone of the HST for preventing a vehicle employing the transaxle apparatus from jerking while the HST is set in the expanded neutral zone.

[0010] Additionally, a center section forming the first and second fluid passages therein is disposed in the housing. In this case, the charge fluid passage may be formed within the center section.

[0011] Also, an oil filter may be disposed in the fluid sump for filtering fluid to be introduced into the charge fluid passage.

[0012] Also, a first check valve, which allows only a flow of fluid from the charge fluid passage to the first fluid passage, may be interposed between the charge fluid passage and the first fluid passage, and a second check valve, which allows only a flow of fluid from the charge fluid passage to the second fluid passage, may be interposed between the charge fluid passage and the second fluid passage.

[0013] A main object of the present invention is to provide such an HST improved in its draining system so that oil drained from the drain oil passage is not easily introduced into the charge fluid passage, thereby preventing the permeation of the insufficiently cooled fluid drained from the at least one of the first and second fluid passages into the closed fluid circuit so as to prevent the closed fluid circuit from overheating.

[0014] To attain the main object, the charge fluid passage and the drain fluid passage are open to the fluid sump while the charge fluid passage and the drain fluid passage being separated from each other. Therefore, the fluid drained from the at least one of first and second fluid passages through the drain oil passage is not directly introduced into the charge fluid passage but is introduced into the fluid sump. Thus, the drained fluid which has been heated by the HST in action is sufficiently cooled in the fluid sump before it is introduced into the charge fluid passage through the fluid sump, thereby keeping fine voluminal efficiency of the HST during its operation and enhancing the endurance of the HST.

[0015] For increasing the distance between both the openings of the charge fluid passage and the drain fluid passage in communication with the fluid sump so as to make the drained fluid further effectively cooled, if the center section is provided, the center section is made to have a first side end and a second side end opposite to each other. An opening of the charge fluid passage in communication with the fluid sump is disposed toward the first side end of the center section, and an opening of the drain fluid passage in communication with the fluid sump is disposed toward the second side end of the center section.

[0016] The opening of the charge fluid passage in communication with the fluid sump may be disposed adjacent to the first side end of the center section. Also, the charge fluid passage may be formed within the center section so as to be disposed adjacent to the first side end.

[0017] The opening of the drain fluid passage in communication with the fluid sump may be disposed adjacent to the second side end of the center section.

[0018] If the opening of the charge fluid passage in communication with the fluid sump is disposed adjacent to the first side end of the center section and the opening of the drain fluid passage in communication with the fluid sump is disposed adjacent to the second side end of the center section, the distance between both the openings can be increased to the limit.

[0019] If the oil filter for filtering fluid introduced into the charge fluid passage is provided, the charge fluid passage is open into communication with the fluid sump inside the oil filter.

[0020] In this case, the drain fluid passage may be open into communication with the fluid sump outside the oil filter so as to increase the distance between both the openings of the charge fluid passage and the drain fluid passage. Also, the opening of the drain fluid passage may be oriented oppositely to the oil filter. If the drain fluid passage allows the backflow of fluid from the fluid sump into said at least one of said first and second fluid passages, the drain fluid passage may be provided with another oil filter between the orifice and the fluid sump. Therefore, the orifice is prevented from being choked by obstacles in the fluid sump. Also, the closed fluid circuit is prevented from being damaged by the obstacles, thereby enhancing the endurance of the HST.

[0021] Alternatively, the drain fluid passage may also be open into communication with the fluid sump inside the oil filter. If the drain fluid passage allows the backflow of fluid from the fluid sump into said at least one of said first and second fluid passages, the oil filter for the charge fluid passage can also be used for filtering fluid to be introduced into the drain fluid passage, thereby reducing the number of parts and the manufacturing costs.

[0022] A second object of the present invention is to provide the HST provided with the above-mentioned improved draining system, which is further improved so as to prevent excessive fluid draining therethrough, thereby keeping the nice voluminal efficiency of the closed fluid circuit during the HST in action.

[0023] To attain the second object, a relief valve is intermediately provided in the drain fluid passage. The relief valve is closed when the hydraulic pressure in the at least one of the first and second fluid passages is increased beyond a predetermined degree. Due to the relief valve, when the HST is operated beyond the limit of the expanded neutral zone so as to increase the hydraulic pressure in the at least one of the first and second fluid passages in connection with the drain fluid passage beyond the predetermined degree, fluid is not drained from the closed fluid circuit of the HST, thereby keeping the voluminal efficiency of the HST so as to keep its nice reaction against a speed controlling operation.

[0024] A third object of the present invention is to provide the HST provided with the above-mentioned improved draining system, which is further improved so as to prevent fluid from back-flowing from the fluid sump into at least one of the first and second fluid passage in connection through the drain fluid passage, thereby preventing the permeation of obstacles in the fluid sump into the orifice and the closed fluid circuit.

[0025] To attain the third object, a third check valve is intermediately provided in the drain fluid passage. The third check valve allows only a flow of fluid from the at least one of the first and second fluid passages to the fluid sump. Even if the at least one of the first and second fluid passages is hydraulically depressed, the third check valve is closed so as to prevent fluid in the fluid sump from being introduced into the at least one of the first and second fluid passages, thereby preventing the closed fluid circuit from being damaged by the obstacles.

[0026] The third check valve is preferably interposed between the orifice and the fluid sump, so as to prevent the orifice from being choked by the obstacles in the fluid sump.

[0027] The closed fluid circuit is supplied with only fluid through either the first or second check valve while the fluid being cleaned by the oil filter.

[0028] Other and further objects, features and advantages of the invention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE FIGURES

[0029]FIG. 1 is a hydraulic circuit diagram of an HST provided with a conventional oil draining system for expanding a neutral zone of the HST.

[0030]FIG. 2 is a diagram of a transaxle apparatus to which the present invention is to be applied.

[0031]FIG. 3 is a plan view of the transaxle apparatus employing the present invention from which an upper housing part 9 t is removed, partly including a sectional view of upper housing part 9 t.

[0032]FIG. 4 is a sectional view taken along lines IV-IV of FIG. 3.

[0033]FIG. 5 is a sectional view taken along lines V-V of FIG. 3.

[0034]FIG. 6 is an enlarged sectional view of a center section 10 to which an oil draining device 101 is fitted according to the present invention.

[0035]FIG. 7 is a hydraulic circuit diagram including oil draining device 101.

[0036]FIG. 8 is an enlarged sectional view of center section 10 to which an alternative oil draining device 102 is fitted according to the present invention.

[0037]FIG. 9 is an enlarged sectional view of center section 10 to which an alternative oil draining device 103 is fitted according to the present invention.

[0038]FIG. 10 is an enlarged sectional view of center section 10 to which an alternative oil draining device 104 is fitted according to the present invention.

[0039]FIG. 11 is an enlarged sectional view of center section 10 to which oil draining device 101 is fitted in a first oil filter 80 according to the present invention.

[0040]FIG. 12 is an enlarged sectional view of center section 10 to which an alternative oil draining device 105 is fitted in a first oil filter 80 according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] Firstly, description will be given on an entire construction of a transaxle apparatus employing an HST as an embodiment of the present invention shown in FIGS. 2 through 5.

[0042] A transaxle apparatus 1 is provided with a housing 9 consisting of an upper housing part 9 t and a lower housing part 9 b joined with each other though a horizontal joint surface. A bearing portion for a motor shaft 22 as discussed below is provided on the joint surface between upper and lower housing parts 9 t and 9 b. A bearing portion for rotatably supporting axles 50L and 50R is disposed in upper housing part 9 t above the joint surface. Inner ends of axles 50L and 50R are differentially connected with each other through a differential gear unit 40 in housing 9. Axles 50L and 50R project laterally from left and right sides of housing 9.

[0043] An interior space of housing 9 is divided into a first chamber R1 and a second chamber R2 by a partition wall 9 i integrally made of housing 9. In first chamber R1 is disposed an HST 8. In second chamber R2 are disposed a drive train 30 including gears for transmitting power from motor shaft 22 to differential gear unit 40, differential gear unit 40 and axles 50L and 50R.

[0044] Partition wall 9 i, in a horizontal way, comprises a lateral portion in parallel to axles 50L and 50R and a longitudinal portion extended perpendicularly to the lateral portion. Both portions are continuously extended from each other, so as to make first chamber R1 adjacent to second chamber R2. In a vertical way, partition wall 9 i is made by two wall portions of upper and lower housing parts 9 t and 9 b, that is, one extended downwardly from the inside surface of the roof wall of upper housing part 9 t to the above-mentioned joint surface, and the other extended upwardly from the inside surface of the bottom wall of lower housing part 9 b to the joint surface. When upper and lower housing parts 9 t and 9 b are joined together, the wall portions are jointed with each other at their ends, so as to form partition wall 9 i which divides the interior of housing 9 into first and second chambers R1 and R2.

[0045] Both first and second chambers R1 and R2 are filled with common oil, thereby serving as oil sumps. As shown in FIG. 4, on the roof wall of housing 9 is provided an oiling cap 95, which is removed when oil is to be poured into housing 9.

[0046] First and second chambers R1 and R2 communicate with each other through an oil filter 81 disposed in an optional portion of partition wall 9 i, so that oil in second chamber R2, which involves obstacles such as iron particles generated from the gears rubbing against one another, is introduced into first chamber R1 through oil filter 81 while being filtered, thereby preventing HST 8 in first chamber R1 from an adverse effect by the obstacles. As a result, oil in housing 9 can be used for both operation of HST 8 in first chamber R1 and lubrication of gears and bearings in second chamber R2 (and first chamber R1).

[0047] On the top wall of upper housing part 9 t corresponding to first chamber R1 is provided an oil circulation port (not shown), which is connected with an external oil reservoir (not shown) through piping such as a rubber hose. Therefore, even if the oil in first chamber R1 is heated so as to increase in volume by driving of HST 8, the excessive oil is made to flow into the external reservoir, thereby regulating the volume of oil in housing 9.

[0048] First chamber R1 occupies an internal area of housing 9 in front of axle 50R and on a lateral side of drive train 30. In first chamber R1 is detachably disposed a center section 10 of HST 8. An elongated portion of center section 10 is oriented in perpendicular to axles 50L and 50R. Center section 10 is provided at the front portion thereof with a vertical surface serving as a motor mounting surface 10 m, onto which a hydraulic motor 21 is mounted. Center section 10 is provided at the rear portion thereof with a horizontal surface serving as a pump mounting surface 10 p, onto which a hydraulic pump 11 is mounted. A pump shaft 12 is centered in pump mounting surface 10 p so as to be vertically supported by center section 10.

[0049] For details about hydraulic pump 11, a cylinder block 14 is slidably rotatably put on pump mounting surface 10 p of center section 10. Cylinder block 14 is provided therein with a plurality of cylinders, into each of which a piston 15 is reciprocally movably inserted through a biasing spring.

[0050] Heads of pistons 15 abut against movable swash plate 13. Pump shaft 12, also serving as an input shaft, is disposed along the rotary axis of cylinder block 14 so as not to be rotatable relative to cylinder block 14. Pump shaft 12 projects upwardly from the top of upper housing part 9 t. A cooling fan 7 and an input pulley 6 are fixed onto the projecting portion of pump shaft 12. As shown in FIG. 2, rotary power of an output shaft 3 of an engine 2 on a vehicle is transmitted into input pulley 6 through an output pulley 4 and a belt 5.

[0051] Movable swash plate 13 is moved so as to slant the surface thereof abutting against pistons 15 at an optional angle from a horizontal surface in perpendicular to the rotary axis of cylinder block 14, thereby changing the direction and amount of oil discharged from hydraulic pump 11.

[0052] For slanting swash plate 13, as shown in FIGS. 2 and 4, a control shaft 60 is rotatably supported in parallel to axles SOL and SOR by a side wall of upper housing part 9 t disposed oppositely to drive train 30 for transmitting power to differential gear unit 40. A basic end of a control arm 61 is fixed onto a portion of control shaft 60 outwardly projecting from the side wall of upper housing part 9 t. A swing arm 62 is attached onto control shaft 60 in housing 9. Control arm 61 interlocks with a manual speed control tool (not shown) such as a lever or a pedal installed adjacently to a seat on the vehicle.

[0053] Swing arm 62 comprises a first arm 62 a and a second arm 62 b both extending radially from control shaft 60. As shown in FIG. 4, an utmost end of first arm 62 a is formed into a projection 63, which is engaged into a groove 13 d provided on a side surface of movable swash plate 13. A phantom center line around which swash plate 13 swings coincides with an axis of control shaft 60, so that, even if swash plate 13 is slanted to any angle, the engagement between projection 63 and groove 13 d is certainly kept.

[0054] When control arm 61 is rotated in the longitudinal direction relative to the vehicle, swing arm 62 is rotated forward or backward around control shaft 60, so as to slant movable swash plate 13 through first arm 62 a, thereby changing the output force of hydraulic pump 11.

[0055] An engaging pin 67 projects from the utmost end of second arm 62 b. In housing 9 is disposed a bush 64 around control shaft 60. A neutral spring 65 of a twisted coil type is wound around bush 64. Both end portions of neutral spring 65 cross each other so as to extend toward second arm 62 b. An eccentric shaft 66 attached to the inside wall of upper housing part 9 t and engaging pin 67 are sandwiched between the both ends of neutral spring 65.

[0056] When control arm 61 is rotated for speed changing operation, one end of neutral spring 65 is outwardly pushed by rotated engaging pin 67 while the other end thereof is stopped by eccentric shaft 66, thereby biasing control arm 61 toward its neutral position. Then, if control arm 61 is released from the operation force, neutral spring 65 returns to its initial state so that engaging pin 67 is forcibly returned to its neutral position demarcated by eccentric shaft 66 and held there.

[0057] Eccentric shaft 66 projects outwardly from housing 9 so as to serve as an adjusting screw. Eccentric shaft 66 is optionally rotated at its adjusting screw portion so as to change in position, thereby accurately adjusting movable swash plate 13 to its neutral position.

[0058] Pressure oil discharged from hydraulic pump 11 is sent to hydraulic motor 21 through a closed fluid circuit in center section 10, thereby constituting HST 8. The closed fluid circuit of HST 8 will be detailed later according to FIGS. 6 and 7.

[0059] Referring to the construction of hydraulic motor 21, as shown in FIG. 3, a cylinder block 24 is slidably rotatably mounted onto vertical motor mounting surface 10 m of center section 10, so that a rotary axis of cylinder block 24 is oriented laterally horizontally. In cylinder block 24 is bored a plurality of cylinders, into each of which a piston 25 is reciprocally movably inserted through a biasing spring.

[0060] Heads of pistons 25 abut against a fixed swash plate 23 fixedly sandwiched between upper and lower housing parts 9 t and 9 b. Motor shaft 22 is laterally horizontally disposed along the rotary axis of cylinder block 24 in parallel to axles 50L and 50R so as not to be rotatable relative to cylinder block 24.

[0061] One end of motor shaft 22 is supported in a bearing hole opening at the center of motor mounting surface 10 m of center section 10. The other end of motor shaft 22 is inserted into second chamber R2 through a bearing 29 on the joint surface of partition wall 9 i. Bearing 29 is provided with a seal for prevention of fluid communication between both chambers R1 and R2 therethrough.

[0062] Description will now be given on drive train 30 for transmitting power from motor shaft 22 into differential gear unit 40.

[0063] An output gear 31 and a brake disk 32, which are integrally formed by a common member, are fixed onto motor shaft 22 in second chamber R2. A brake device 33 for applying force on brake disk 32 is disposed adjacent to brake disk 32 so as to brake motor shaft 22.

[0064] As shown in FIG. 3, brake device 33 comprises brake pads 36 and 36′, a brake control shaft 34 and a brake arm (not shown). Brake pads 36 and 36′ are disposed adjacent to brake disk 32. Brake control shaft 34 is formed into a sectionally D-like shaped cam for pushing brake pad 36 against brake disk 32. Brake control shaft 34 is vertically supported by the roof wall of upper housing part 9 t. A top end of brake control shaft 34 projects upwardly from housing 9 so as to be fixedly provided thereon with the brake arm.

[0065] When brake arm 35 is rotationally operated, brake control shaft 34 is rotated integrally with brake arm 35 so that its cam presses brake pad 36 against brake disk 32, thereby generating frictional force to brake motor shaft 22.

[0066] A counter shaft 39 is horizontally supported behind motor shaft 22 in parallel. A diametrically small gear 38 is freely provided on counter shaft 39. A diametrically large gear 37, which is provided along its inner periphery with teeth coinciding with teeth of gear 38, is pierced by gear 38 through the inner peripheral teeth so that gears 37 and 38 are not rotatable relative to each other. Gear 37 engages with output gear 31 fixed on motor shaft 22.

[0067] A ring gear 41 of differential gear unit 40 constantly engages with gear 38.

[0068] Referring to the construction of differential gear unit 40 as shown in FIG. 3, in ring gear 41 are disposed a pair of pinion shafts 42 in perpendicular to axles 50L and 50R, so that pinion shafts 42 are rotated integrally with ring gear 41. A pair of pinions 43 as bevel gears are rotatably provided on respective pinion shafts 42. A pair of differential side gears 44 are fixed onto respective inner ends of axles 50L and 50R, so as to engage with respective pinions 43. Accordingly, the power input into ring gear 41 is shared by axles 50L and 50R through pinions 43 and differential side gears 44.

[0069] Next, description will be given on the closed hydraulic circuit formed in center section 10 of HST 8.

[0070] Center section 10 comprises a first side end 10 a and a second side end 10 b opposite to each other. A pair of parallel first horizontal oil hole 71 and second horizontal oil hole 72 are straightly bored into center section 10 from a surface of first side end 10 a toward the opposite surface of second side end 10 b along the longitudinal direction of center section 10. First and second horizontal oil holes 71 and 72 are juxtaposed in a vertical row so that first horizontal oil hole 71 is disposed above second horizontal oil hole 72. The open ends of first and second horizontal oil holes 71 and 72, which are open at the first side surface of center section 10, are plugged by plugs 75 and 76, respectively. The other ends of first and second horizontal oil holes 71 and 72 in vicinity of the second side surface are closed within center section 10. A slant oil hole 73 is branched from an intermediate portion of second horizontal oil hole 72. An open end of oil hole 73 is plugged by a plug 74.

[0071] Each of pump and motor mounting surfaces 10 p and 10 m of center section 10 is provided with a pair of first and second kidney ports 91 and 92. Both first kidney ports 91 of pump and motor mounting surfaces 10P and 10 m are joined to first oil hole 71. Thus, both first kidney ports 91 are connected with each other through first oil hole 71, thereby constituting a first oil passage 93. Second kidney port 92 of pump mounting surface 10 p is joined to slant oil hole 73. Second kidney port 92 of motor mounting surface 10 m is joined to second horizontal oil hole 72. Both second kidney ports 92 are connected with each other through second horizontal oil hole 72 and slant oil hole 73, thereby constituting a second oil passage 94. Hydraulic pump 11 and hydraulic motor 21 are fluidly connected with each other through a closed circuit comprising first and second oil passages 93 and 94.

[0072] A vertical hole is bored through center section 10 adjacent to the surface of first side end 10 a together with plugs 75 and 76, thereby providing a charge oil passage 82. A pair of check valves 83 are disposed in respective plugs 75 and 76, so that one of check valves 83 is interposed between first horizontal oil hole 71 and charge oil passage 82, and the other between second horizontal oil hole 72 and charge oil passage 82. Check valves 83 are opened by hydraulic depression of respective oil passages 93 and 94 so as to allow oil in charge oil passage 82 to flow into respective oil passages 93 and 94 and prevent the oil flowing from respective oil passages 93 and 94 into charge oil passage 82. A bottom open end of charge oil passage 82 serves as a charge oil inlet 84, which is covered with a first oil filter 80 as shown in FIGS. 4 and 5.

[0073] When the manual speed control tool is operated for forward traveling, hydraulic pump 11 acts so as to hydraulically pressurize first oil passage 93 and relatively hydraulically depress second oil passage 94, thereby driving hydraulic motor 21 in a forward traveling mode. When the manual speed control tool is operated for backward traveling, hydraulic pump 11 acts so as to hydraulically pressurize second oil passage 94 and relatively hydraulically depress first oil passage 93, thereby driving hydraulic motor 21 in a backward traveling mode. During these actions of hydraulic pump 11, oil is absorbed through charge oil inlet 84 into charge oil passage 82 while being filtered by first oil filter 80, and is introduced into hydraulically depressed one of first and second oil passages 93 and 94 through corresponding check valve 83, thereby compensating the oil leak from the closed fluid circuit of HST 8.

[0074] The closed fluid circuit of HST 8 is provided with an oil draining device serving as a relief valve, which releases oil in second oil passage 94 for expanding the neutral zone of hydraulic pump 11, in other words, for keeping HST 8 neutral while movable swash plate 13 is slightly slipped from its proper neutral position into its backward traveling range. Description will now be given on some embodiments of center section 10 to which an oil draining device is fitted for expanding the neutral zone of HST 8.

[0075] Referring to FIGS. 5, 6 and 8-10, center section 10 is provided with a vertical drain port 111 joined to a closed end portion of second horizontal oil hole 72, which is disposed adjacent to second side end 10 b.

[0076] Referring to each of FIGS. 5, 6 and 8-10, each of various oil draining devices 101-104 is mounted onto the bottom of center section 10 adjacent to second side end 10 b of center section 10 so as to cover the bottom opening of drain port 111. As mentioned above, charge oil inlet 84 also disposed on the bottom of center section 10 is adjacent to first side end 10 a. Therefore, the oil draining device is disposed oppositely to charge oil inlet 84 with respect to center section 10 or second oil hole 72. As a result, the oil draining device is sufficiently spaced from charge oil inlet 84 so as to be prevented from the adverse effect of oil charging.

[0077] Referring to FIGS. 5, 6 and 7, oil draining device 101 comprises an upwardly opening cylindrical holder 112. Holder 112 is screwed upwardly into center section 10 so as to cover the open bottom end of drain port 111, whereby the interior space of holder 112 communicates with drain port 111. In holder 112 are disposed a vertically slidable valve sleeve 114 and a biasing spring 113. A stopper ring 115 is engaged in holder 112 so as to limit the upward motion of valve sleeve 114.

[0078] Valve sleeve 114 is bored by at least one horizontally radial orifice 116 having an extremely small diameter which communicates inside and outside with respect to valve sleeve 114. Valve sleeve 114 is provided along its outer periphery with a loop groove 117 joined to orifice 116. Holder 112 is bored by a radial drain hole 118, which is outwardly open at the outer peripheral surface of holder 112 so as to be brought into communication the oil sump in first chamber R1. The opening of drain hole 118 is horizontally directed toward a downward extension of second side end 10 b of center section 10 and oppositely to first oil filter 80. The vertical position of drain hole 118 is determined so as to communicate with loop groove 117 of valve sleeve 114 when valve sleeve 114 reaches its upper limit position. Thus, drain port 111, orifice 116, loop groove 117 and drain hole 118 constitutes a drain oil passage.

[0079] The upward biasing force of spring 113 is predetermined enough to keep valve sleeve 114 in contact with stopper ring 115 despite a slight increase of hydraulic pressure in second oil passage 94 over its neutral level, thereby holding orifice 116 in communication with drain hole 118 through loop groove 117. Hence, excessive oil in second oil passage 94 is drained to the oil sump in housing 9 through orifice 116, loop groove 117 and drain hole 118, thereby keeping the hydraulic pressure in second oil passage 94 at its neutral level. Therefore, even if, while the manual speed control tool is located at its neutral position, swash plate 13 undergoes a slight slippage from the proper neutral position into its backward traveling range because of inaccurate connection between swash plate 13 and the manual speed control tool, the neutral condition of HST 8 is kept so as to prevent the vehicle from unexpected driving.

[0080] Description will now be given on the difference of oil draining system between the conventional type shown in FIG. 1 and the present embodiment shown in FIGS. 6 and 7. Referring to FIG. 1, a drain oil passage having an orifice 116 is disposed adjacent to check valves 83. If the hydraulic pressure in second oil passage 94 becomes slightly higher than its desired neutral level because of inaccurate neutral location of movable swash plate 13 of hydraulic pump 11, heated oil is drained to a charge oil passage 82 through the drain oil passage bypassing one check valve 83, however, the drained oil is likely to be absorbed soon into hydraulically depressed first oil passage 93 through the other check valve 83, thereby increasing the oil temperature in the whole of the closed fluid circuit in center section 10.

[0081] On the other hand, referring to the present invention shown in FIGS. 6 and 7, drain hole 118 is disposed apart from charge oil inlet 84 of charge oil passage 82. The excessive oil in second oil passage 94 is drained from drain hole 118 into the oil sump in housing 9, and then, the closed circuit of HST 8 is supplied with oil from the oil sump through first oil filter 80, charge oil inlet 84 and charge oil passage 82, thereby preventing the closed fluid circuit of HST 8 from overheating.

[0082] When the manual speed control tool is operated for backward traveling of the vehicle so that hydraulic pump 11 is actuated to increase the hydraulic pressure of second oil passage 94 over the biasing force of spring 113, valve sleeve 114 is made to slide downwardly by the increased hydraulic pressure so as to cut the communication between loop groove 117 and drain hole 118, thereby preventing second oil passage 94 from oil leak. Accordingly, acting HST 8 is prevented from reduction of its voluminal efficiency, thereby keeping its fine reaction to manual speed changing operation.

[0083] Referring to FIG. 8, an oil draining device 102 is equal to oil draining device 101 additionally provided with a check valve 120 comprising a ball 121 and a stopper ring 122. Ball 121 is contained in valve sleeve 114. Stopper ring 122 having an inner periphery which is diametrically smaller than ball 121 is fixed onto the inner wall of valve sleeve 114.

[0084] If the manual speed control tool is operated for forward traveling of the vehicle, hydraulic pump 11 acts so as to hydraulically depress second oil passage 94. However, second oil passage 94 is not supplied with oil in housing 9 through drain hole 118 and orifice 116 because ball 121 is raised by hydraulic depression of second oil passage 94 so as to close the hole of stopper ring 122. In other words, oil in housing 9 is not directly fed into second oil passage 94 without passing through first oil filter 80, so that obstacles involved in the oil sump of first chamber R1 neither enter the closed fluid circuit of HST 8 nor choke orifice 116.

[0085] When the manual speed control tool is located in neutral or operated for backward traveling of the vehicle, ball 121 descends under its own weight into contact with the top of a stopper 123 uprightly provided in valve sleeve 114, thereby making second horizontal oil hole 72 communicate with orifice 116 of valve sleeve 114. Accordingly, in quite the same way with oil draining device 101 of the first embodiment, during the neutral positioning of the manual speed control tool, oil draining device 101 serving as a relief valve is opened so as to drain oil from second oil passage 94, thereby preventing the vehicle from creeping. During the backward traveling operation, the oil draining passage in oil draining device 102 is closed so as to hold the hydraulic pressure in second oil passage 94, thereby preventing HST 8 in action from reduction of its voluminal efficiency.

[0086] Referring to an alternative oil draining device 103 shown in FIG. 9, a holder 112′ is mounted onto center section 10 adjacent to second side end 10 b. Holder 112′ is vertically bored through by a hole in connection with drain port 111. An upper portion of the vertical through-hole is radially narrowed so as to serve as an orifice 116′. A lower portion of the same is radially widened so as to serve as a valve chamber 125. Valve chamber 125 is open at its bottom to the oil sump in housing 9. Some members are provided into valve chamber 125, thereby constituting a check valve 120′. A stopper ring 127 is fixedly provided along the peripheral surface of valve chamber 125 in vicinity of its bottom opening. A valve plate 126 is fixedly interposed between stopper ring 127 and holder 112′ in valve chamber 125. A ball 121 is vertically movably disposed in valve chamber 125 above valve plate 127.

[0087] A ceiling of valve chamber 125 is formed in a conical shape, thereby serving as a valve seat for fitting with ball 121.

[0088] During the forward driving operation of the manual speed control tool, ball 121 is raised by the hydraulic depression of second oil passage 94 so as to abut against the conical ceiling surface of valve chamber 125, thereby closing orifice 116′ so as to prevent second oil passage 94 from the backflow of oil from the oil sump in housing 9, which involves obstacles as a cause of reducing the efficiency of HST 8 and choking orifice 116′. On the other hand, check valve 83 is opened by the hydraulic depression of second oil passage 94 so as to introduce the oil from the oil sump in housing 9 cleaned by first oil filter 80 into second oil passage 94, thereby compensating the oil loss of the closed-fluid circuit of HST 8.

[0089] Referring to an alternative oil draining device 104 shown in FIG. 10, holder 112′ provided therein with orifice 116′ and valve chamber 125 is identical with that of oil draining device 103, however, check valve 120′ is removed and valve plate 126 is replaced with a second oil filter 130.

[0090] Due to the removal of check valve 120′, during the forward driving operation, the oil of oil sump in housing 9 is also introduced through orifice 116′ into hydraulically depressed second oil passage 94 while being cleaned by second oil filter 130, thereby enhancing the oil compensation of the closed fluid circuit of HST 8.

[0091] Above-mentioned four oil draining devices 101-104 are classified into two groups: one is provided with a check valve for preventing the backflow of oil into second oil passage 94 hydraulically depressed during forward traveling; and the other is not provided with such a check valve. Oil draining devices 102 and 103 are provided with respective check valves 120 and 120′. Each of oil draining devices 101 and 104 is not provided with a check valve so as to allow oil to backflow from the oil sump in housing 9 into hydraulically depressed second oil passage 94 during the forward traveling operation. Furthermore, oil draining device 104 can filter the back-flow oil through its second oil filter 130.

[0092] In the case of allowing the backflow of oil without a check valve, an oil draining device is preferably provided with a strainer such as oil filter 130. However, if the oil draining device is disposed between second oil passage 94 in center section 10 and first oil filter 80, an additional strainer is unnecessary, thereby reducing the number of parts and the manufacturing costs.

[0093] Then, as shown in FIGS. 11 and 12, center section 10 may be alternatively provided therein with a drain port 111′ which is disposed near charge oil passage 82 so as to be open in cylindrical first oil filter 80 while being spaced from charge oil passage 82 as far as possible.

[0094] Referring to FIG. 11, oil draining device 101 is disposed in first oil filter 80 and mounted onto center section 10 so as to communicate with drain port 111′. The opening of drain hole 118 bored in holder 112 is oriented toward the inner surface of first oil filter 80 opposite to charge oil inlet 84, thereby making the oil drained from drain hole 118 into the oil sump in first oil filter 80 as hard as possible to be introduced into charge oil inlet 84.

[0095] During the forward traveling of a vehicle, in addition to check valve 83 connected to first horizontal oil hole 71, the drain oil passage formed in oil draining device 101 allows oil to flow therethrough into hydraulically depressed second oil passage 94 while being cleaned by first oil filter 80.

[0096] Referring to FIG. 12, an oil draining device 105 is equal to simplified oil draining device 104, from which second oil filter 130 is removed and a portion forming valve chamber 125 therein is cut away, due to first oil filter 80. Oil draining device 105 is constituted by only a holder 112″ forming an orifice 116″ therein.

[0097] While second oil passage 94 is hydraulically pressured higher than its neutral level, orifice 116″ allows oil to flow therethrough to the oil sump in first oil filter 80, thereby keeping the neutral condition of HST 8 or serving as a relief valve during the backward traveling of a vehicle. During the forward traveling of a vehicle, in addition to check valve 83 connected to first horizontal oil hole 71, orifice 116″ allows oil to flow therethrough into hydraulically depressed second oil passage 94 while being cleaned by first oil filter 80, thereby supplying the closed fluid circuit of HST 8 with operating oil.

[0098] Oil draining device 102 provided with check valve 120 or oil draining device 103 provided with check valve 120′ may be disposed in first oil filter 80 so as to be connected to drain port 111′.

[0099] For expanding the neutral zone of HST 8, any of oil draining devices 101-105 may be alternatively connected to first oil passage 93. In this case, swash plate 13 is allowed to slip slightly from its proper neutral position into its forward traveling range while the manual speed control tool in connection with swash plate 13 is set at its neutral position. Furthermore, both first and second oil passages 93 and 94 may be provided with respective oil draining devices so as to expand a dead zone of swash plate 13 for keeping HST 8 neutral into both its forward and backward ranges.

[0100] It is further understood by those skilled in the art that the foregoing description is a preferred embodiment of the disclosed device and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof. 

What is claimed is:
 1. A hydrostatic transmission, comprising: a housing, an interior space of said housing serving as a fluid sump; a pair of first and second fluid passages disposed in said housing; a hydraulic pump disposed in said housing; a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit as a hydrostatic transmission; a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit, and a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage and said drain fluid passage are open to said fluid sump while said charge fluid passage and said drain fluid passage being separated from each other.
 2. The hydrostatic transmission as set forth in claim 1, further comprising: a relief valve intermediately provided in said drain fluid passage, wherein said relief valve is closed when hydraulic pressure in said at least one of said first and second fluid passages in connection with said drain fluid passage is increased beyond a predetermined degree.
 3. The hydrostatic transmission as set forth in claim 1, further comprising: a check valve intermediately provided in said drain fluid passage, wherein said check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.
 4. The hydrostatic transmission as set forth in claim 3, wherein said check valve is interposed between said orifice and said fluid sump.
 5. The hydrostatic transmission as set forth in claim 1, further comprising: an oil filter interposed between said orifice of said drain fluid passage and said fluid sump.
 6. The hydrostatic transmission as set forth in claim 1, further comprising: a center section having a first side end and a second side end opposite to each other disposed in said housing, said center section forming said first and second fluid passages therein, wherein an opening of said charge fluid passage in communication with said fluid sump is disposed toward said first side end of said center section, and wherein an opening of said drain fluid passage in communication with said fluid sump is disposed toward said second side end of said center section.
 7. The hydrostatic transmission as set forth in claim 6, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section.
 8. The hydrostatic transmission as set forth in claim 7, wherein said charge fluid passage is formed within said center section so as to be disposed adjacent to said first side end.
 9. The hydrostatic transmission as set forth in claim 6, wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 10. The hydrostatic transmission as set forth in claim 6, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section, and wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 11. A hydrostatic transmission, comprising: a housing, an interior space of said housing serving as a fluid sump; a pair of first and second fluid passages disposed in said housing; a hydraulic pump disposed in said housing; a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit as a hydrostatic transmission; a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit; a first oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage, and a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage is open into communication with said fluid sump inside said first oil filter and said drain fluid passage is open into communication with said fluid sump outside said first oil filter while said charge fluid passage and said drain fluid passage being separated from each other.
 12. The hydrostatic transmission as set forth in claim 11, further comprising: a relief valve intermediately provided in said drain fluid passage, wherein said relief valve is closed when hydraulic pressure in said at least one of said first and second fluid passages in connection with said drain fluid passage is increased beyond a predetermined degree.
 13. The hydrostatic transmission as set forth in claim 11, further comprising: a check valve intermediately provided in said drain fluid passage, wherein said check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.
 14. The hydrostatic transmission as set forth in claim 13, wherein said check valve is interposed between said orifice and said fluid sump.
 15. The hydrostatic transmission as set forth in claim 11, further comprising: a second oil filter interposed between said orifice of said drain fluid passage and said fluid sump.
 16. The hydrostatic transmission as set forth in claim 11, wherein said drain fluid passage is oriented oppositely to said first oil filter.
 17. The hydrostatic transmission as set forth in claim 11, further comprising: a center section having a first side end and a second side end opposite to each other disposed in said housing, said center section forming said first and second fluid passages therein, wherein said first oil filter is disposed toward said first side end of said center section, so that an opening of said charge fluid passage in communication with said fluid sump is disposed toward said first side end of said center section and an opening of said drain fluid passage in communication with said fluid sump is disposed toward said second side end of said center section.
 18. The hydrostatic transmission as set forth in claim 17, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section.
 19. The hydrostatic transmission as set forth in claim 18, wherein said charge fluid passage is formed within said center section so as to be disposed adjacent to said first side end.
 20. The hydrostatic transmission as set forth in claim 17, wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 21. The hydrostatic transmission as set forth in claim 17, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section, and wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 22. A hydrostatic transmission comprising: a housing, an interior space of said housing serving as a fluid sump; a pair of first and second fluid passages disposed in said housing; a hydraulic pump disposed in said housing; a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit; a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit; an oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage, and a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein both said charge fluid passage and said drain fluid passage are open into communication with said fluid sump inside said oil filter while said charge fluid passage and said drain fluid passage being separated from each other.
 23. The hydrostatic transmission as set forth in claim 22, further comprising: a relief valve intermediately provided in said drain fluid passage, wherein said relief valve is closed when hydraulic pressure in said at least one of said first and second fluid passages in connection with said drain fluid passage is increased beyond a predetermined degree.
 24. The hydrostatic transmission as set forth in claim 22, further comprising: a check valve intermediately provided in said drain fluid passage, wherein said check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.
 25. The hydrostatic transmission as set forth in claim 24, wherein said check valve is interposed between said orifice and said fluid sump.
 26. The hydrostatic transmission as set forth in claim 22, further comprising: a center section having a first side end and a second side end opposite to each other disposed in said housing, said center section forming said first and second fluid passages therein, wherein said oil filter is disposed toward said first side end of said center section, so that an opening of said charge fluid passage in communication with said fluid sump is disposed toward said first side end of said center section and an opening of said drain fluid passage in communication with said fluid sump is disposed toward said second side end of said center section.
 27. The hydrostatic transmission as set forth in claim 26, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section.
 28. The hydrostatic transmission as set forth in claim 27, wherein said charge fluid passage is formed within said center section so as to be disposed adjacent to said first side end.
 29. A hydrostatic transmission comprising: a housing, an interior space of said housing serving as a fluid sump; a pair of first and second fluid passages disposed in said housing; a hydraulic pump disposed in said housing; a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit; a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit; a first check valve interposed between said charge fluid passage and said first fluid passage, wherein said first check valve allows only a flow of fluid from said charge fluid passage to said first fluid passage; a second check valve interposed between said charge fluid passage and said second fluid passage, wherein said second check valve allows only a flow of fluid from said charge fluid passage to said second fluid passage; a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage and said drain fluid passage are open into communication with said fluid sump while said charge fluid passage and said drain fluid passage being separated from each other; and a relief valve provided in said drain fluid passage, wherein said relief valve closes when hydraulic pressure in said at least one of said first and second fluid passages is increased beyond a predetermined degree.
 30. The hydrostatic transmission as set forth in claim 29, further comprising: a third check valve intermediately provided in said drain fluid passage, wherein said third check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.
 31. The hydrostatic transmission as set forth in claim 30, wherein said third check valve is interposed between said orifice and said fluid sump.
 32. The hydrostatic transmission as set forth in claim 29, further comprising: a center section having a first side end and a second side end opposite to each other disposed in said housing, said center section forming said first and second fluid passages therein, wherein an opening of said charge fluid passage in communication with said fluid sump is disposed toward said first side end of said center section, and wherein an opening of said drain fluid passage in communication with said fluid sump is disposed toward said second side end of said center section.
 33. The hydrostatic transmission as set forth in claim 32, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section.
 34. The hydrostatic transmission as set forth in claim 33, wherein said charge fluid passage and said first and second check valves are formed within said center section so as to be disposed adjacent to said first side end.
 35. The hydrostatic transmission as set forth in claim 32, wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 36. The hydrostatic transmission as set forth in claim 32, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section, and wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 37. The hydrostatic transmission as set forth in claim 29, further comprising: a first oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage, wherein said charge fluid passage is open into communication with said fluid sump inside said first oil filter and said drain fluid passage is open into communication with said fluid sump outside said first oil filter.
 38. The hydrostatic transmission as set forth in claim 37, wherein an opening of said drain fluid passage into communication with said fluid sump is oriented oppositely to said first oil filter.
 39. The hydrostatic transmission as set forth in claim 37, further comprising: a second oil filter interposed between said orifice of said drain fluid passage and said fluid sump.
 40. The hydrostatic transmission as set forth in claim 29, further comprising: an oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage, wherein both said charge fluid passage and said drain fluid passage are open into communication with said fluid sump inside said oil filter.
 41. A hydrostatic transmission, comprising: a housing, an interior space of said housing serving as a fluid sump; a pair of first and second fluid passages disposed in said housing; a hydraulic pump disposed in said housing; a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit; a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit; a first check valve interposed between said charge fluid passage and said first fluid passage, wherein said first check valve allows only a flow of fluid from said charge fluid passage to said first fluid passage; a second check valve interposed between said charge fluid passage and said second fluid passage, wherein said second check valve allows only a flow of fluid from said charge fluid passage to said second fluid passage; a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage and said drain fluid passage are open into communication with said fluid sump while said charge fluid passage and said drain fluid passage being separated from each other; and an oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage, wherein said charge fluid passage is open into communication with said fluid sump inside said oil filter and said drain fluid passage is open into communication with said fluid sump outside said oil filter.
 42. The hydrostatic transmission as set forth in claim 41, further comprising: a valve provided in said drain fluid passage, wherein said valve closes when hydraulic pressure in said at least one of said first and second fluid passages is increased beyond a predetermined degree.
 43. A hydrostatic transmission, comprising: a housing, an interior space of said housing serving as a fluid sump; a pair of first and second fluid passages disposed in said housing; a hydraulic pump disposed in said housing; a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit as a hydrostatic transmission; a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit, wherein charge fluid flows from a charge fluid inlet into a first valve, then into the charge fluid passage, and into a second valve; and a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage and said drain fluid passage are open to said fluid sump while said charge fluid passage and said drain fluid passage are separated from each other.
 44. The hydrostatic transmission as set forth in claim 43, further comprising: a relief valve intermediately provided in said drain fluid passage, wherein said relief valve is closed when hydraulic pressure in said at least one of said first and second fluid passages in connection with said drain fluid passage is increased beyond a predetermined degree.
 45. The hydrostatic transmission as set forth in claim 43, further comprising: a check valve immediately provided in said drain fluid passage, wherein said check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.
 46. The hydrostatic transmission as set forth in claim 45, wherein said check valve is interposed between said orifice and said fluid sump.
 47. The hydrostatic transmission as set forth in claim 43, further comprising: an oil filter interposed between said orifice of said drain fluid passage and said fluid sump.
 48. The hydrostatic transmission as set forth in claim 43, further comprising: a center section having a first side end and a second side end opposite to each other disposed in said housing, said center section forming said first and second fluid passages therein, wherein an opening of said charge fluid passage in communication with said fluid sump is disposed toward said first side end of said center section, and wherein an opening of said drain fluid passage in communication with said fluid sump is disposed toward said second side end of said center section.
 49. The hydrostatic transmission as set forth in claim 48, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section.
 50. The hydrostatic transmission as set forth in claim 49, wherein said charge fluid passage is formed within said center section so as to be disposed adjacent to said first side end.
 51. The hydrostatic transmission as set forth in claim 48, wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 52. The hydrostatic transmission as set forth in claim 48, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section, and wherein said opening of said drain fluid in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 53. A hydrostatic transmission, comprising: a housing, an interior space of said housing serving as a fluid sump; a pair of first and second fluid passages disposed in said housing; a hydraulic pump disposed in said housing; a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit as a hydrostatic transmission; a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit, wherein charge fluid flows from a charge fluid inlet into a first valve, then into the charge fluid passage, and into a second valve; a first oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage; and a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage is open into communication with said fluid sump inside said first oil filter and said drain fluid passage is open into communication with said fluid sump outside said first oil filter while said charge fluid passage and said drain fluid passage are separated from each other.
 54. The hydrostatic transmission as set forth in claim 53, further comprising: a relief valve intermediately provided in said drain fluid passage, wherein said relief valve is closed when hydraulic pressure in said at least one of said first and second fluid passages in connection with said drain fluid passage is increased beyond a predetermined degree.
 55. They hydrostatic transmission as set forth in claim 53, further comprising: a check valve intermediately provided in said drain fluid passage, wherein said check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.
 56. The hydrostatic transmission as set forth in claim 53, wherein said check valve is interposed between said orifice and said fluid sump.
 57. The hydrostatic transmission as set forth in claim 53, wherein said drain fluid passage is oriented oppositely to said first oil filter.
 58. The hydrostatic transmission as set forth in claim 53, further comprising: a center section for having a first side end and a second side end opposite to each other disposed in said housing, said center section forming said first and second fluid passages therein, wherein said first oil filter is disposed toward said first side end of said center section, so that an opening of said charge fluid passage in communication with said fluid sump is disposed toward said first side end of said center section and an opening of said drain fluid passage in communication with said fluid sump is disposed toward said second side end of said center section.
 59. The hydrostatic transmission as set forth in claim 58, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section.
 60. The hydrostatic transmission as set forth in claim 59, wherein said charge fluid passage is formed within said center section so as to be disposed adjacent to said first side end.
 61. The hydrostatic transmission as set forth in claim 58, wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 62. The hydrostatic transmission as set forth in claim 58, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 63. A hydrostatic transmission comprising: a housing, an interior space of said housing serving as a fluid sump; a pair of first and second fluid passages disposed in said housing; a hydraulic pump disposed in said housing; a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit; a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit, wherein charge fluid flows from a charge fluid inlet into a first valve, then into the charge fluid passage, and into a second valve; a first check valve interposed between said charge fluid passage and said first fluid passage, wherein said first check valve allows only a flow of fluid from said charge fluid passage to said first fluid passage; a second check valve interposed between said charge fluid passage and said second fluid passage, wherein said second check valve allows only a flow of fluid from said charge fluid passage to said second fluid passage; a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage and said drain fluid passage are open into communication with said fluid sump while said charge fluid passage and said drain fluid passage are separated from each other; and a relief valve provided in said drain fluid passage, wherein said relief valve closes when hydraulic pressure in said at least one of said first and second fluid passages is increased beyond a predetermined degree.
 64. The hydrostatic transmission as set forth in claim 63, further comprising: a third check valve intermediately provided in said drain fluid passage, wherein said third check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.
 65. The hydrostatic transmission as set forth in claim 63, wherein said third check valve is interposed between said orifice and said fluid sump.
 66. The hydrostatic transmission as set forth in claim 63, further comprising: a center section having a first side end and a second side end opposite to each other disposed in said housing, said center section forming said first and second fluid passages therein, wherein an opening of said charge fluid passage in communication with said fluid sump is disposed toward said first side end of said center section, and wherein an opening of said drain fluid passage in communication with said fluid sump is disposed toward second side end of said center section.
 67. The hydrostatic transmission as set forth in claim 66, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section.
 68. The hydrostatic transmission as set forth in claim 67, wherein said charge fluid passage and said first and second check valves are formed within said center section so as to be disposed adjacent to said first side end.
 69. The hydrostatic transmission as set forth in claim 66, wherein said opening of said drain fluid passage in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 70. The hydrostatic transmission as set forth in claim 66, wherein said opening of said charge fluid passage in communication with said fluid sump is disposed adjacent to said first side end of said center section, and wherein said opening of said drain fluid passages in communication with said fluid sump is disposed adjacent to said second side end of said center section.
 71. The hydrostatic transmission as set forth in claim 63, further comprising: a first oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage, wherein said charge fluid passage is open into communication with said fluid sump inside said first oil filter and said drain fluid passage is open into communication with said fluid sump outside said first oil filter.
 72. The hydrostatic transmission as set forth in claim 61, wherein an opening of said drain fluid passage into communication with said fluid sump is oriented oppositely to said first oil filter.
 73. The hydrostatic transmission as set forth in claim 63, further comprising: an oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage, wherein both said charge fluid passage and said drain fluid passage are open into communication with said fluid sump inside said oil filter. 